Why are parallel lines important?

Parallel lines are important in mathematics because they are at the base of several conjectures involving angles in geometry. Drawing a line, called a transversal, through a pair of parallel lines forms three different types of angles that have known mathematical properties.

The three types of angles formed from a transversal and a set of parallel lines are corresponding angles, alternate interior angles and alternate exterior angles. Each angle in a set is congruent to the other angles of the same type. Therefore, if the measure of one angle is known, it is possible to find the measure of other angles.

Parallel lines in geometry are lines in the same plane that have the same slope and do not intersect.